Recently, for the purpose of further improving the efficiency of a refrigeration cycle apparatus, there is proposed a power-recovery type refrigeration cycle apparatus using an expansion mechanism instead of an expansion valve in which the expansion mechanism recovers the pressure energy as power in the course of the expansion of a refrigerant (working fluid), thereby reducing the electric power required for driving a compression mechanism by the amount of the power recovered. Such a refrigeration cycle apparatus uses an expander compressor unit in which a motor, a compression mechanism and an expansion mechanism are coupled by a shaft.
In the expander compressor unit, the compression mechanism and the expansion mechanism are coupled by the shaft, and therefore there may be a case where the displacement of the compression mechanism is insufficient, or the displacement of the expansion mechanism is insufficient, depending on the operational conditions. In order to ensure recovery power even under operational conditions where the displacement of the compression mechanism is insufficient so that the COP (Coefficient of Performance) of the refrigeration cycle apparatus can be kept high, there also is proposed a refrigeration cycle apparatus using a secondary compressor in addition to an expander compressor unit (see, for example, Patent literature 1). In this refrigeration cycle apparatus, the secondary compressor is operated so that the high pressure in a refrigeration cycle should be a specified target value.
FIG. 8 is a configuration diagram indicating a refrigeration cycle apparatus described in Patent literature 1. As indicated in FIG. 8, the refrigeration cycle apparatus using an expander compressor unit 220 and a second compressor 230 includes a refrigerant circuit 210 and a controller 250 as a control device. In the refrigerant circuit 210, a first compression mechanism 221 of the expander compressor unit 220 and a second compression mechanism 231 of the second compressor 230 are disposed in parallel between an indoor heat exchanger 211 and an outdoor heat exchanger 212. Further, the first compression mechanism 221 is coupled with a motor 222 and an expansion mechanism 223 by a shaft, and the second compression mechanism 231 is coupled with a motor 232 by a shaft.
The controller 250 controls the second compressor 230 so that the high pressure in a refrigeration cycle should be a specified target value. Specifically, if the measured value of the high pressure Ph is higher than the target value, the controller 250 reduces the discharge amount from the second compression mechanism 231 by decreasing the rotation speed of the motor 232, and if the measured value of the high pressure Ph is lower than the target value conversely, it increases the discharge amount from the second compression mechanism 231 by increasing the rotation speed of the motor 232.
Accordingly, even under operational conditions where the displacement only of the first compression mechanism 221 is insufficient, it is possible to compensate for the shortage of the displacement by driving the second compression mechanism 231. Thus, the operation of the refrigeration cycle apparatus can be continued with a high COP.
Meanwhile, for higher output of a refrigeration cycle apparatus, there also is a refrigeration cycle apparatus using a plurality of compressors. For example, Patent literature 2 discloses a refrigeration cycle apparatus as indicated in FIG. 9. This refrigeration cycle apparatus includes a refrigerant circuit 310 in which two compressors 320 and 330 are disposed in parallel. Oil to be used for lubricating and sealing the sliding portions of the compression mechanism is stored inside the compressors 320 and 330. Such a refrigeration cycle apparatus has problems in the context of reliability and efficiency if the amount of the oil stored in each of the compressors 320 and 330 is unbalanced. To solve the problems, the refrigeration cycle apparatus disclosed in Patent literature 2 employs a structure for balancing the amount of oil to be stored in the two compressors 320 and 330.
That is, as indicated in FIG. 9, pipes on the refrigerant-discharge side of the compressors 320 and 330 each are provided with an oil separator 311 and an oil-bypass pipe 312 extending from the oil separator 311 to each pipe on the refrigerant-suction side of the compressors 320 and 330. Further, as indicated in FIG. 10, the lower portions of the compressors 320 and 330 are coupled to each other by an oil-equalizing pipe 350, allowing oil to flow between the compressors 320 and 330 through the oil-equalizing pipe 350. Further, a pipe on the high-pressure side of the refrigeration cycle is provided with a pressure sensor 315.
During operation of the two compressors 320 and 330, the following operation is carried out as an oil-equalizing operation.
First, the operation frequency of the one compressor 320 is stepped up by a particular value, and the operation frequency of the other compressor 330 is decreased until a set time ta has elapsed so that the pressure Pd detected by the pressure sensor 315 does not vary. After the set time ta has elapsed, the operation frequency of the one compressor 320 is stepped down by a particular value, and the operation frequency of the other compressor 330 is increased until a set time ta has elapsed in the same manner so that the pressure Pd detected by the pressure sensor 315 does not vary. Then, after the set time ta has elapsed again, the operation frequency of the compressors 320 and 330 is returned. After every passage of the set time period tb, the above-mentioned oil-equalizing operations of step up and step down are repeated.
Thus, by coupling the compressors 320 and 330 using the oil-equalizing pipe 350 as well as varying the operation frequency of the compressors 320 and 330 alternately during operation of the two compressors 320 and 330, the oil of the compressors 320 and 330 is allowed to flow through the oil-equalizing pipe 350 efficiently, so that the amount of oil to be stored in each of the compressors 320 and 330 is balanced.